add capabilities to TraVarT and restructure interfaces

[JakobGretenkort]

important changes:

• added a common ILanguage interface above the IPlugin interface to gather methods that all languages share
• added a language implementation for UVL
• renamed reader/writer to deserializer/serializer and changed their interfaces to better reflect the new role
• changed the UVLReader and UVLWriter implementations to fit the new deserializer/serializer interfaces
• added a IPrettyPrinter interface for producing human-readable representations of variability models
• implemented default pretty printer for all languages